According to the known procedures, PCMM is produced by chlorination of carbon disulfide in the presence of small quantities of iodine, about 0.005 to 2% (based on weight of carbon disulfide) as catalyst, according to the overall reaction I: EQU cs.sub.2 + 3cl.sub.2 .fwdarw. CCl.sub.3 SCl + SCl.sub.2 (I)
this reaction theoretically consumes 409 g of carbon disulfide and 1.145 grams of chlorine per kilogram of PCMM formed, corresponding to a molar ratio Cl.sub.2 /CS.sub.2 equal to 3.0. However, in industrial practice the chlorine is charged in an amount considerably smaller than theoretical, the molar ratio Cl.sub.2 /CS.sub.2 scarcely exceeding 2.50 in order not to risk destruction of part of the formed PCMM in the following well-known side reactions: EQU CCl.sub.3 SCl + Cl.sub.2 .fwdarw. CCl.sub.4 + SCl.sub.2 (II) EQU ccl.sub.3 SCl + SCl.sub.2 .fwdarw. CCl.sub.4 + S.sub.2 Cl.sub.2 (III)
simultaneously reation (IV), forming carbon tetrachloride directly by the action of chlorine and carbon disulfide, can take place, particularly in the presence of light and traces of metallic chlorides: EQU CS.sub.2 + 3Cl.sub.2 .fwdarw. CCl.sub.4 + S.sub.2 Cl.sub.2 (IV)
the extent of this reaction corresponds to at least 5% of the charged carbon disulfide. After separating the low-boiling products by distillation (preferably at reduced pressure), a minimum of 1.5% of sulfur monochloride remains, being difficult to remove because its boiling point (138.degree.C) is so close to that of PCMM (148.degree.C.)
In practice, then, equation V represents the overall reaction which takes place with at best 95% yield based on carbon disulfide charged: EQU 3CS.sub.2 + 7.5Cl.sub.2 .fwdarw. 2.5 CCl.sub.3 - SCl + 2.5 SCl.sub.2 + 0.5 CS.sub.2 (V)
under these conditions, the consumption of reactants is raised from theoretical to at least 508 grams of carbon disulfide and 1.188 grams of chlorine per kilogram of technical grade of PCMM, 98.5% pure.
In most of the prior processes, the higher boiling PCMM and sulfur monochloride are isolated by distillation under reduced pressure being delivered at the tail end or foot of the distillation unit while the lower boiling fractions comprising carbon disulfide, sulfur dichloride and carbon tetrachloride are delivered at the head of the distillation unit and are then transformed into a mixture of carbon tetrachloride and sulfur monochloride by chlorination according to the overall equation VI, using a metallic chloride as catalyst: EQU 2.5 SCl.sub.2 + 0.5 CS.sub.2 + 0.25 Cl.sub.2 .fwdarw. 0.5 CCl.sub.4 + 1.75 S.sub.2 Cl.sub.2 (VI)